Intermediate frequency amplifier



Dec. 3, 1940. P. F. G. HoLsT ETAL.

INTERMEDIATE FREQUENCY AMPLIPIER Filed .nine 18, 195s Jan/04915' 2 Patented Dec. 3, 1940 PATENT OFFICE INTERMEDIATE FREQUENCY AMPLIFIER Paul F. G. Holst and Loren R. Kirkwood, Oaklyn,

N. J., assgnors to Radio Corporation of America, a. corporation of Delaware Application June 18, 1938, Serial No'. 214,471

14 Claims.

The present invention relates to .an intermediate frequency amplifier for a high fidelity (adjustable selectivity) superheterodyne radio receiving system, and has for its object to provide a coupling network in an intermediate frequency amplifier, wherein the selectivity characteristic may be adjusted while at thesarne time a sharp peak may be provided in said characteristic for all degrees of selectivity adjustment, by simple switching means, whereby tuning of a receiving system embodying said ampliiier may be facilitated. n

It is also an object of the present invention to provide an intermediate frequency amplifier having an improved interstage coupling network adapted for a wide range of selectivity control, and means for imparting thereto a sharp tuning response for all degrees of selectivity controllable through said network.

It is also a further object of the present invention to provide an improved tuning indication system for a radio receiver embodying a selectivity control network in an intermediate frequency amplier which isv adapted for simple switching control for changing the shape of the response characteristics of said amplier whereby a peak is provided for accurate tuning, and means in said system responsive to signals for indicating said peak tuning.

The invention will, however, be better understood from the following description when considered in connection with the accompanying drawing and its scope will be pointed out in the appended claims.

In the drawing: i

Figure 1 is a schematic circuit diagram of a radio receiving system provided with an intermediate frequency amplier embodying the invention, f

Figures 2 and 3 are curves illustrating operating characteristics of the amplier shown in Fig. 1, and

Figure 4 is a schematic `circuit diagram showing a modication of a portion of the circuit of Fig. 1. l

Referring to Fig. 1, a radio receiving system is shown comprising the usual radio frequency detector and oscillator tuning system indicated in the rectangle 5 and provided with a manual or other suitable tuning control element 6. The tuning system supplies intermediate frequency signals to an intermediate frequency amplifier tube 'I which is coupled through a coupling network 8 with a second intermediate frequency amplifier tube 9 which, in turn, is coupled through (Cl. Z50- 20) a suitable tuned intermediate frequency output transformer IU with the second detector I I.

Rectified signals from the detector appear across the output resistor I2, supplied through a resistor network I3, I4 and I5 to a suitable audio frequency amplifier (not shown), an automatic volume control circuit I6, and a tuning indicator tube I1. 'I'he latter is of the cathode ray grid controlled type, such as an RCA 6E5 tube in which the variable D. C. potential derived from the output resistor I2 in response to signals, controls the potential on a control element I8 which lies in the path of electrons flowing irom'a cathode I9 onto a fluorescent screen 2l). As this type of tube is well known and represents any suitable tuning indicating device providing an accurate tuning indication, further description is unnecessary. I

Signals applied to the intermediate frequency amplier are subjected to the control oi the network 8 which imparts to the amplifier a predetermined selectivity characteristic, and is adjustable to provide differing degrees of selectivity in any desired number of steps in each of which a sharp peak may be produced to provide a sharp tuning response.

The coupling network is arranged to provide the sharp response or peak referred to for tuning purposes regardless of the selectivity adjustment for normal reception of signals, and comprises a transformer having three windings 22, 23 and 24 arranged in axially parallel relation to each other substantially as indicated, to provide electro-magnetic coupling between the coils or windings 22 and 23 and electromagnetic coupling between the coils 23 and 24 as indicated. The inductance of each of the windings is adjustable by means of magnetite movable cores indicated at 25 and each winding is tuned to substantially the same intermediate frequency by shunt tuning capacitors 26. A resistor 2l connected between the third winding 24 and the tuning capacitor 26 therefor is normally connected in series with the tuned circuit as shown, but may be snorted out l by a switch 28 for reasons which will hereinafter appear.

The second winding 23 is connected in circuit with a fourth winding 30 and a switch3l providing additional electromagnetic coupling between the circuit including the coil 23 and the rst coil 22, the degree of coupling being controlled by the switch 3I. In the position shown, the switch 3| is connected to eliminate the winding 30 from the circuit of the second coil 23 and provides two additional coupling steps including a portion and all of the coil 3G in circuit with the winding 23 and the shunt capacitor 26, and correspondingly decreased selectivity for the coupling network.

The winding 22 and shunt capacitor 26 provides a tuned output anode circuit for the first amplier 1 and may be considered as the primary winding of the transformer. The winding 23 and the tuning capacitor 26 therefor together with the winding 30, when in circuit, provides an intermediate secondary or tuned link circuit for transferring energy from the input or primary winding 22 to the output or tertiary winding 24 which is connected to the grid circuit 0f the second amplier 9.

The transformer arrangement shown is known as a three-circuit intermediate frequency coupling transformer having input, intermediate, and output tuned circuits and additional variable or adjustable coupling between the intermediate and input circuits and resistance damping for the output circuit cooperating to yvary the selectivity characteristic of the network in a predetermined manner. This characteristic with resistance in the tertiary or output circuit is as.

necessary, ordinarily, to increase the selectivity by reducing the coupling through the switch 3|, to provide the response characteristic shown by the curve 35 in order that the tuning indicator tube may indicate accurately an exact resonance with a received signal. By damping the circuit by resistance means as shown, the desirableA response characteristic for signal `reception with.

high iidelity is provided.

As shown in Fig. 3, it has been found that the curves 36 and 31 may be made to assume a sharp peak corresponding to the peak of the curve 35, by controlling the same damping resistance 21 provided in the tertiary circuit for the broad tun-` ing response. For example, when the switch 28 is closed to short circuit the resistor 21 inthe tertiary circuit, the response characteristic assumes the peaked form shown in Fig. 2 for the three degrees of selectivity shownA by the curves. It will be seen that all of thev curves have a peak characteristic 38 providing 35, 36 and 31.

sharp tuning indication at resonancey without necessitating changing the selectivity characteristic of the interstage networkby operation of the switch 3|.

The switch 28 may be a simple, single` point.v switch which may be moved to the closed position for tuning, by a suitable connection with the tuning control element 6 as indicated by the dotted line 39 in Fig. 1. The` intermediate circuit is grounded as indicated at 29, and the resistor 21 is short circuited by grounding the resistor at the opposite end through a single circuit connection 30. With this arrangement, the resistor 21 is momentarily shorted out during the tuning.

operation to provide the sharp tuning response, and the overall selectivity adjustment remains unchanged in whatever position it is adjusted for. The response is then restored to normal as shown in Fig. 3 upon releasing the tuning control ele-.;

ment by causing the switch 28 to open.

The coupling network shown may be applied between other amplifier stages in a similar manner, the single coupling network being shown in the present example in order to simplify the drawing and the description.

Referring to Fig. 4, along with the preceding gures, the circuit of Fig. 1 may be modified as indicated, to provide a switch 34 corresponding to the switch 3| in the number of contacts or operating positions and conjointly controllable therewith to connect the lead 33 to ground for the position of the switch 3| corresponding to the highest degree of selectivity.

This arrangement in effect is the same as permanently closing the switch 28 and making it unnecessary to operate the switch 28 when tuning when the highest degree of selectivity is desired. This is possible for the reason that the curve 35 in Fig. 3 is only slightly broader than the same curve in Fig. 2, thereby providing substantially the same fidelity.

While three degrees of selectivity are provided inthe system shown, additional contacts may be provided with suitable connections to coupling windings in the coil 30 to increase the number of steps in the selectivity adjustment. Where a .greater number than that shown are provided in the circuit, it is desirable to utilize the circuit of Fig. 4 vfor the most selective position of the control switch in order to eliminate the necessity for actuating the switch 28 when manually tuning for distant stations.

We claim as our invention:

l. In a superheterodyne radio receiving system, thecornbination with tuning means therefor, of means for visually indicating peak resonance to received signals, means for adjusting the selectivity' characteristic of said system whereby the fidelity of response to received signals is controlled, and means for imparting a single relatively sharp resonance peak to said selectivity characteristic for actuating said resonance indicating means while maintaining the adjustment of said second named means and the overallselectivity characteristic provided thereby substantially xed.

2.`,In a superheterodyne radio receiving systern,v the'ccmbination with tuning means therefor, of means for visually indicating peak resotrance to received signals, means for adjusting the selectivity characteristic of said system whereby the i'ldelity of response to received signals 'is controlled, and means for imparting a single relatively sharp resonance peak to said selectivity characteristic for actuating said-resonance indicating means while maintaining the adjustment of' said third named means and the overall'selectivity characteristic provided thereby substantially fixed, said last-named means being manually controllable with said tuning means. Y

3. In a high fidelity superheterodyne radio receiving system, the combination of lmeans for adjusting the selectivity characteristic ofv said system whereby the fidelity of response to received signalsiis controlled, means for imparting substantially a flat-top selectivity characteristic tonsaid system for high fidelity operation, and .means operative when tuning said system to provide a single peak inl the hat top of said last-named characteristic while maintaining the adjustment of said first named means and the overallselectivity characteristic provided thereby substantially xed. l

li-InA a high'fidelity superheterodyne radio receiving system, the combination of an intermediate frequency coupling network including means for adjusting the selectivity characteristic of said system whereby the fidelity of response to received signals is controlled, meansI in said network for imparting a predetermined selectivity characteristic to said system for high fidelity operation, and means in said network operative when tuning said system to provide a single resonance peak in said last named characteristic while maintaining the adjustment of said first named means and the overall selectivity characteristic provided thereby substantially iiXedffor improved tuning response. y

5. In a high fidelity superheterodyne radio receiving system, the combination of a kthree-circuit interstage coupling transformer having an input, intermediate and output tuned circuits through which signals are conveyed inthe order named, means for controlling the coupling between the intermediate circuit and the input circuit and the selectivity characteristic of said system, whereby the fidelity of response to received signals is controlled, `means providing `damping resistance in the .output circuit only and conjoint control of said fidelity to impart substantially a flat-top to said selectivity characteristic, and means for reducing the damping resistance in. the output circuit to provide a single reso- `nance peak in said response characteristic while maintaining the adjustment of the overall band width of the selectivity characteristic for improved tuning response.

6. In a superheterodyne radio receiving system,

ithe combination with tuning means therefor, of

means for indicating resonance to received signals, means for adjusting the selectivity of said system and means for imparting a relatively sharply peaked tuning response to said system for actuating said indicating means while maintaining the overall band width of the selectivity adjustment of said system, comprising a threecircuit intermediate frequency coupling transformer having an input circuit, an intermediate circuit and an output circuit responsive to substantially the same intermediate frequency, a coupling winding for increasing the coupling between the input and intermediate circuits and broadening the selectivity characteristic of said system, a .selector switch for selectively including said winding and portions thereof in the intermediate circuit, a damping resistor in said output circuit for further controlling said selectivity characteristic, and a switch conjointly operable with said tuning means for reducing the resistance in said output circuit to provide a peak in the selectivity characteristic when tuning.

7. In an intermediate frequency amplier, an interstage coupling network comprising a threecircuit intermediate frequency coupling transformer having an input, an output and an intermediate Winding, a circuit for each winding tuned to substantially the same intermediate frequency, a damping resistor in circuit with the output winding only and means controlling the coupling between the circuit of the intermediate winding and the input winding for broadening the response characteristic of the network in predetermined degrees with substantially a flat top, and means for imparting a single peak to the top of said response characteristic comprising a control element for said resistor for reducing the resistance in circuit with said output winding to a predetermined degree.

8. In a radio receiving system, the combination with tuning means and a tuning indicator device for indicating correct tuning or resonance to a received carrier wave, of an intermediate frequency amplifier, an interstage coupling network therefor, comprising a three-circuit intermediate frequency transformer having a tuned primary winding, a tuned secondary winding and a tuned tertiary winding through which signals are transferred in the order named, means providing adinstable coupling between the primary winding and the intermediate winding and a plurality of differing degrees of selectivity, means for damping the tuned circuit of the Vtertiary winding only for enhancing the selectivity characteristic, and means controlling said damping means when tuning to impart asingle sharp tuning peak toy ing a plurality of differing degrees of selectivity, means providing a damping resistance in the tuned circuit of the tertiary winding only for en- 1302 winding and the intermediate winding provid-Y hancing the selectivity characteristic, and means 51 for controlling the amount of resistance in circuit to impart a single sharp tuning peak to the selectivity response characteristic of said amplifier for all conditions of selectivity adjustment of said amplifier.

l0. In an intermediate frequency amplifier, the combination of an interstage coupling network, comprising a three-circuit intermediate frequency transformer having a tuned primary winding, a tuned secondary winding and a tuned tertiary winding through which signals are transferred in the order named by electromagnetic coupling, means providing additional controllable electromagnetic coupling between the primary winding and the intermediate winding providing a plurality of differing degrees of selectivity, means providing a damping resistance in the tuned circuit of the tertiary winding for enhancing the selectivity characteristic, and means for controlling the amount of resistance in circuit to impart a single sharp tuning peak to the selectivity response characteristic of said amplifier for all conditions of selectivity adjustment of said amplifier, said last named means comprising a switch connected with the tuning control element and operable to close during the tuning operation thereof to short circuit said damping resistance.

11. The combination with an intermediate frequency amplifier, of an interstage coupling network comprising a three-circuit intermediate frequency coupling transformer having an input, an output and an intermediate winding, a circuit for each winding tuned to substantially the same intermediate frequency, a damping resistor in circuit with the output winding only and means for increasing the coupling between the circuit of the intermediate winding and the input winding for broadening the response characteristic of the network in predetermined degrees with substantially a ilat top response characteristic, means for imparting a, single resonance peak to said response characteristic including a control element for said resistor for 5 reducing the resistance in circuit with said output winding,'a tuning system in connection with said amplifier for applying intermediate frequency signals thereto and having a tuning element for actuating said control element when tuning, and means connected with said amplifier and including a tuning indicator tube responsive to variations in signal amplitude for indicating resonance to received signals.

l2. In a radio receiving system, the combination with a movable. tuning element and a tuning indicator tube responsive to variations in a variable controlling potential, of a signal rectifier for applying'said controlling potential to said tube, an intermediate frequency amplier for received signals connected'with said rectier, an interstage coupling network in said amplifier comprising a three-circuit transformer having a primary, intermediate and output tuned circuits each including a transformer winding, means providing 'a controllable coupling between the primary and intermediate circuits including a coupling winding in the intermediate circuit coupled electromagnetically with the primary Winding, a selector switch connected with said 30.?y coupling Winding for including the coupling Winding and portions thereof selectively in circuit, and means for damping the tuned output circuit conjointly with adjustment of said selector switch.

13. In a radio receiving system, the combination with a movable tuning element and a tuning indicator responsive to variations in a variable controlling potential, of a signal rectifier for applying said controlling potential to said indicator, an intermediate frequency amplifier for received signals connected with said rectifier, an interstage coupling network in said amplifier comprising a three-circuit transformer having a primary, intermediate and output tuned circuits each including a transformer winding, means providing a controllable coupling between the primary and 5 intermediate circuits including a coupling Winding in the intermediate circuit coupled electromagnetically with the primary winding, a selector switch connected with said coupling winding for including the coupling winding and portions 10 thereof selectively in circuit, means for control ling the damping of said output circuit conjointly operable with and by movement of said tuning element, and additional means for controlling the damping of said output circuit when said 15 selector switch is adjusted for a condition of maximum selectivity.

i4. In a radio receiving system, the combination with a movable tuning element and a tuning indicator responsive to variations in a vari- 20 able controlling potential, of a signal rectier for applying said controlling potential to said indicator, an intermediate frequency amplifier for received signals connected with said rectifier, an interstage coupling network in said amplifier 25 comprising a three-circuit transformer having a primary, intermediate and output tuned circuits each including a transformer winding, means providing a controllable coupling between the primary and intermediate circuits including a coupling winding in the intermediate circuit coupled electromagnetically with the primary winding, a selector switch connected with said coupling winding for including the coupling winding and portions thereof selectively in circuit, and means for controlling the damping of said output circuit conjointly operable with and by movement of said tuning element.

PAUL F. G. HoLslT. LOREN R. IURKWOOD. 

